Potentiation of chemically induced cleft palate by ethanol ingestion during gestation in the mouse

Multiple chemical exposures: synergism vs. individual exposure levels

Exposure to single chemicals is known to produce congenital malformations in both pregnant animals and humans exposed at sufficiently high intensity. However, real life involves multiple, simultaneous exposures. Using as a database the 43 multiple chemical exposure studies located by Nelson (Teratology 49:33-71; 1994) where synergism was reported, we explored the degree to which such concerns may be realistic from the viewpoint of the current standard developmental toxicity safety evaluation process. Focusing on the assessment of the lowest tested dose of a given agent participating in synergistic activity as compared to its threshold level for eliciting toxicity when administered alone, we found that while the availability of adequate data was limited, all cases, with the possible exception of one, demonstrated synergistic toxic expression only when at least one, and usually both, compounds were used at or above their individual threshold for toxicity. These findings suggest that in animals such phenomena of synergistic chemical interactions are likely to occur only when at least one and more likely both agents are administered at or above their individual threshold for toxicity. To the extent animal studies are predictive of human developmental hazards due to single chemical exposures, available data do not establish multiple chemical exposures as a major human developmental concern.

Developmental effects of combined exposure to ethanol and vitamin A

The potential for ethanol (EtOH) to influence the developmental toxicity of vitamin A was investigated. 11 groups of approximately 31 FDA-bred Osborne-Mendel rats received either a control or isocaloric 6.4% EtOH liquid diet (containing 4000 IU vitamin A/litre) ad lib. The vehicle control, EtOH and pair-fed (pair-fed against the EtOH group) groups received corn oil (the vehicle) by gavage. Vitamin A was administered by gavage without EtOH at 40,000, 80,000, 120,000 or 160,000 IU/kg daily. Vitamin A was administered by gavage at 10,000, 20,000, 40,000 or 80,000 IU/kg with EtOH ad lib., daily throughout the study. Combined EtOH and vitamin A resulted in significant reductions in maternal diet consumption and body weight when doses of vitamin A were as low as 10,000 IU/kg. The most severe effects on overall (days 0-20) maternal body weight gain were observed in the groups receiving 120,000 or 160,000 IU vitamin A/kg alone or EtOH in combination with 80,000 IU vitamin A/kg. The overall diet consumption (days 0-20) paralleled the overall weight gain. In general, pups exposed to ethanol and vitamin A had a tendency to weigh less than those exposed to vitamin A alone, but to weigh more than those exposed to EtOH alone. EtOH combined with vitamin A at 80,000 IU/kg resulted in an increased incidence of cleft palate relative to the vehicle control or either treatment alone. The incidence of exencephaly and protruding tongue was significantly greater in the group given vitamin A at 160,000 IU/kg, compared with the vehicle control group. The most consistent statistically significant skeletal finding in the groups receiving combined treatment was a treatment-related increased incidence of supernumerary ribs [14th rib (C7), 14th rib bud (L1) and 15 ribs]. In addition, the incidence of misshapen zygomatic arch was also significantly increased in the group exposed to EtOH and vitamin A at 80,000 IU/kg. The incidence of moderately enlarged renal pelvis and severely enlarged ureter proximal to the kidney was increased in the group exposed to EtOH and vitamin A at 80,000 IU/kg relative to the vehicle control, or either treatment alone. Therefore, for some of the endpoints examined in this investigation, it would appear that ethanol potentiates the developmental effects of vitamin A.

Interactions in developmental toxicology: a literature review and terminology proposal

Developmental toxicologists have investigated the interactive effects from concurrent exposures to a variety of chemical and physical agents, including therapeutic drugs, industrial agents, and some biological organisms or their toxins. Of approximately 160 reports of concurrent exposures reviewed in this paper, about one third report no interactive effects (including additive effects--usually referring to response--as opposed to dose-additivity); another one third report antagonistic effects, and the final third report potentiative or synergistic effects. The quality of the studies is highly variable. Frequently, only small numbers of animals were included, and very few dose levels were evaluated. Maternal toxicity was rarely discussed. Time-effect relationships were examined infrequently. In addition, these studies are also inconsistent in the use of terms to describe interactive effects, and more than 90% of the terms were not in harmony with currently accepted definitions in toxicology. Because interaction studies will continue to be important in the future, this paper proposes uniform usage of terms for additivity and interactions in developmental toxicology: additivity (the combined effect of two or more developmental toxicants approximates the sum of the effects of the agents administered separately); antagonism (the combined effect of two or more agents, one or more of which are present at doses that would be developmentally toxic if given individually, is significantly less than the sum of the effects of the agents administered separately); potentiation (the increased effect of a developmental toxicant by concurrent action of another agent at a dose that is not developmentally toxic); synergism (the combined effect of two or more developmental toxicants is significantly greater than the sum of the effects of each agent administered alone); and, interaction if more precise terminology does not apply.